- LMAXADU = 40000 ....but sky background is 49000. (Wow! How bright is your sky?) You have exceeded the linear limit for every pixel. This entire image is not usable for photometry.

Why no calculation of air mass? You have in the FITS header:

DATE-OBS

2012-06-24T00:40:54

[ISO 8601] UTC date/time of exposure start

JD

2456102.5284027779

Julian Date at start of exposure

SITELAT

40 28 05

SITELONG

-03 42 24

OBJCTRA

14 58 11.30

OBJCTDEC

+40 40 19.6

CENTAZ

288.9901

Nominal Azimuth of center of image in deg

CENTALT

48.8787

Nominal Altitude of center of image in deg

...this should be enough to determine air mass...and the values appear reasonable...Bootes is up in your western sky at this time of night.

You may have to contact Geir about this issue. I do not know if VPHOT is unable to read the formatted data in your FITS header.

It appears your sky is very bright. If that's the case, and with a 20cm aperture telescope...I recommend you work on brighter targets that require shorter exposures. (When I examine the MK421 image and call up the AAVSO comparison stars....many of the brighter ones are unusable because they are above the non-linear limit. Therefore you can only use fainter comparison stars. And the sky is very bright...which means you have a smaller range of stars that you can measure...and they are at lower SNR because the bright sky adds much noise to the measure of the stars. Find targets that are brighter by at least two magnitudes, and use exposures that are shorter.